The present invention relates generally to spring testing apparatus, and more particularly to apparatus and methods for testing heavy duty compression and tension springs, bellows and like resilient members.
Compression and tension springs of pre-selected size, length and force are essential to the efficient, safe and optimum performance of most machines and equipment, and matched or "mated" springs having substantially equal characteristics are designated in the standards and specifications of equipment manufacturers. In the automotive field, for instance, matched sets of springs are essential to the proper performance of internal combustion engines, brake systems and the like. Spring manufacturers generally are able to achieve a substantial uniformity in matching spring sets having essentially identical characteristics (within close tolerances) through internal quality control of manufacturing and testing procedures, but even new springs are often mismatched and/or fail to meet the prescribed specifications established for the equipment in which these springs are used.
The problems of unequal or mismatched springs become even more acute through actual usage of the equipment, and resultant uneven wear, damage, fatigue or similar problems in the springs or the equipment parts operated thereby. In automotive brake systems, brake shoe return springs (see FIG. 6) and brake chamber retracting springs (see FIG. 7) can become weak and unmatched due to elongation ("spring stretch"), fatigue or the like, and this may cause erratic performance, slow brake release, uneven wear or uneven brake application and excessive heat buildup. Brake lining wear and unmated springs produce pulling or dragging brake problems and infrequent brake adjustments may result in excessive stretching of the return springs and permanent elongation thereof. These springs also function (stretching and contracting) in the confined heat area of the drum and excessive stretch and heat may produce metal fatigue in the springs. Properly functioning, shoe return springs should react quickly and evenly so that both brakes on the same axle will be balanced. Besides being mated for equal tension, the springs should have the tension of new springs conforming to equipment manufacturers specifications. For instance, the largest manufacturers of transit and highway buses, trucks and tractors use heavy duty Rockwell front and rear axles and mechanical brake systems. The GMC brake specifications for Rockwell axles require front brake shoe return springs (see FIG. 5) having a free length of 8 11/16 inches and a stretch or tension length of 9 13/32 inches with a "pounds-pull" tension of 32 to 38 pounds. The GMC brake specifications for rear brake shoe retraction springs (see FIG. 4) call for a free length of 81/2 inches and a stretch length of 9 13/32 inches with a 113 to 137 pound tension. It may be noted that the specified stretch length of both front and rear brake springs is identical despite substantial differences in the relative strength and free length of these springs.
In the servicing of such heavy duty truck brakes or the like, the length and tension of all return springs must be checked and matched pairs of correct springs installed on both brakes of the same axle to assure efficient and safe brake operation. Similarly, the compressive force of conventional brake chamber springs is vital to brake balance since even small variations in tension or force are magnified in operation due to the compounding factors of leverage in driving the slack adjusters to rotate the camshaft for applying the brake shoes. Therefore, the brake chamber springs for both actuators must be checked and mated to each other within the length and tension specifications for proper functioning of both brakes on the same axle.
It will be understood that the necessity for mating brake spring sets to meet the requirements just given for heavy duty air brake systems of commercial buses and heavy over-the-road trucks is equally applicable to the vacuum-actuated hydraulic or straight hydraulic brake systems of medium to light trucks, buses, automobiles and other vehicles and implements in general.
In the past, helical-wound tension springs (single and double coil) have traditionally been tested, if at all, by elementary "fish scale" type devices which themselves are spring loaded and subject to spring elongation and fatigue thereby compounding errors and variations in the test results. Over-reaction to such simplistic methods has resulted in complex, delicate instrumentation and complicated procedures for testing tension and/or compression springs. In short, heretofore there has been no simple, positive acting, spring testing apparatus or method for quickly and easily comparing and mating either compression or tension springs into matched sets.